The present invention relates to a printing bridge for silk-screen printing machines, of the type comprising at least one mobile transversal support beam and one or more elements acting to retain a doctor mounted in a sliding way on the same transversal support beam.
Generally, silk-screen printing machines comprise a lower frame designed to support and operate the surface to be printed, such as sheets for the printing of posters for instance, a semi-permeable elastic form located in correspondence of the surface, and an upper printing bridge equipped with a mobile transversal beam to which doctor retaining elements are linked in a sliding way. During printing, the doctor is pressed onto the inked semi-permeable form against the surface to be printed and is moved with respect to the form by the transverse beam, to allow the deposition of the ink on the same surface. This set of operations is almost totally automated in modern silk-screen printing machines.
In particular, the retaining elements of the doctor could comprise pneumatic actuators to lock jaws on the same doctor and/or to press the doctor against the semi-permeable form. Such actuators, which move with the transverse beam of the printing bridge, are connected to a source of air under pressure by means of flexible tubes of sufficient length to extend for the full travel of the same transversal beam during printing.
Furthermore, the retaining elements, usually two or four in number, can slide along the transverse beam, and can be locked in position, to allow the printing bridge to adapt to the different dimensions of the surfaces to be printed. In fact, as the width of the surface to be printed varies, the size of the semi-permeable form and of the doctor must also vary, and the mutual distance between the retaining elements needs to change accordingly to effectively retain and operate the doctor during the printing.
In particular, this sliding of the retaining elements of the doctor along the transverse support beam is effected manually since it is not simple to produce an automatic means for the movement of the retaining elements. The mutual interference between the means of movement of the different retaining elements, together with the crossover of the same means of movement with the flexible compressed air distribution tubes, makes it difficult to employ automatic means to move such retaining elements.
The manual change of the doctor retaining elements involves considerable interruption of the printing activity and requires the presence of a skilled fitter who (with the machine stopped) arranges the same elements with precision in a predefined position on the transverse beam.
One purpose of the present invention is to furnish a printing bridge for silk-screen printing machines, either automatic or semiautomatic, which easily allows the employment of computer-controlled motive means to move the retaining elements along the transverse beam. Another purpose of the present invention is to furnish a printing bridge that allows the precise positioning of the doctor along the transverse beam and reduces the setup time of the machine during the job changeover.
And a further purpose of the present invention is to furnish a printing bridge that is simple to produce and adaptable to different demands of printing jobs.
These and other purposes are achieved by the printing bridge for silk-screen printing machines according to the first independent claim and the following dependent claims.
The printing bridge for silk-screen printing machines, according to the present invention, comprises a mobile transversal support beam and at least one doctor retaining and/or adjusting element, mounted in a sliding way on the same transverse support beam. The retaining and/or adjusting element, furthermore, comprises at least one portion of connection to motive means for sliding on the transverse support beam and the latter comprises at least a section bar having at least one seat for the relative sliding of the portion of connection and/or of the motive means.
According to an advantageous embodiment of the present invention, the retaining and/or adjusting element comprises at least one fluid-driven actuator and the section bar of the printing bridge is endowed with one or more longitudinal ducts for the transport of a driving fluid for the actuator. Furthermore, the section bar also presents transversal tubes to distribute the driving fluid to the retaining element.
According to a further aspect of the present invention, the portion of connection to the motive means of each retaining element comprises a threaded hole and, in their turn, the motive means comprise a threaded driving rod that engages the threaded hole. The printing bridge, furthermore, comprises at least two retaining elements, each of which is endowed with a perforated portion through which the threaded driving rod of the other retaining element can pass.
In a particular form of embodiment, the printing bridge comprises four retaining and/or adjusting elements and the transverse support beam presents two joined section bars, substantially mirror images of each other.